This invention relates to rotation monitors for monitoring rotations of a rotatable element and, more particularly, for countering changes in conditions or disturbances related to such a rotation monitor during its operation. Rotation measurement devices of various kinds have been used for monitoring the motion of rotating elements through detecting change or changes in one or more parameters characterizing the motion of such rotating element during rotations thereof. In one kind of arrangement based on detection of optical parameter changes, such a rotating element has a relatively large electromagnetic radiation reflectance surface portion and a relatively low electromagnetic radiation reflectance surface portion both encountered along a circular concentric band therein to provide detectable optical parameter changes during each rotation. Alternatively, the surface may instead have an electromagnetic radiation transmissive portion and a radiation nontransmissive portion encountered along that circular band to provide the desired detectable optical parameter changes.
One well known device in which there is a desire to monitor rotations of a rotating member is an electric power meter, or watt-hour meter, which has a rotating disk, or eddy wheel, which is often arranged to rotate about a vertical axis and so in a horizontal plane during use. Since the meter is constructed to have the rotations of this eddy wheel proportional to the electrical power consumed, the number of rotations must be counted as a measure of that power consumption. Rotation monitors provided for this purpose can accomplish this monitoring by emitting electromagnetic radiation to impinge on a circular concentric band on the surface on one side of the wheel which is substantially reflective of such radiation therealong except where interrupted by a low reflective marking. The emitted radiation is reflected from the band during rotation of the wheel, except from the low reflectivity mark or except from an alternative opening in the wheel, to a photodetector on that side of the wheel. Alternatively, such electromagnetic radiation can be emitted on one side of such a wheel to impinge on a circular concentric band in the facing surface thereof, which is opaque except where one or more openings are provided in the wheel along the band, this radiation to be detected by a photodetector on the opposite side of the wheel whenever such openings pass between the emitter and the detector.
The resulting electrical signals from the photodetector output have varying magnitudes therein with larger values corresponding to the times when such electromagnetic radiation is detected and lower values corresponding to those times when it is not detected. This output signal is provided to signal processing circuitry to obtain from these varying magnitudes the number of rotations of the disk, and so the amount of electrical power consumed. The use of an additional emitter at a location differing from the first emitter, or of an additional photodetector at a location differing from the first photodetector, permits determining not only the number of rotations of the eddy wheel but also the direction of such rotations which is useful in those situations in which power from the load is transmitted back to the source or in which some kinds of physical tampering with the metering system are desired to be detected. Such information derived by the signal processing circuitry may be further communicated to remote locations using suitable communication techniques.
One such meter system monitor uses two light emitting diodes (LED's) capable of emitting electromagnetic radiation in the near infrared portion of the spectrum with such emitted radiation being reflected from the meter eddy wheel to a photodetector is located approximately between these emitters to receive same. A view of such an arrangement is shown in FIG. 1 in which a housing, 10, has the two LED's, 11 and 12, housed therein so as to be capable of directing infrared electromagnetic radiation emitted therefrom along corresponding paths, 13 and 14, shown in dashed lines, to a surface, 15, on a side of an eddy wheel, 16, of which a portion is shown. Surface 15 is supported and rotated with respect to LED's 11 and 12 so as to have a circular concentric band therein continually intercepting paths 13 and 14 during rotations of wheel 16 from which surface band the emitted radiation is substantially reflected except at a low reflectivity mark, 17, positioned across a portion of that band. Housing 10 also houses a photodetector, 18, which receives much of any such reflected infrared electromagnetic radiation from emitters 11 and 12 over corresponding return paths, 19 and 20, also shown in dashed lines beginning from the same circular concentric band.
An operation controller and signal processing system, 21, receives the electrical signals corresponding to the reflected electromagnetic radiation impinging on photodetector 18 over an interconnection path, 22. This system is also coupled to LED's 11 and 12 over corresponding interconnections, 23 and 24, to permit the controller therein to cause them to emit infrared electromagnetic radiation pulses in a sequence over time formed by the repeating pulse pattern at a repetition rate sufficient to detect the passage of low reflectivity mark 17 at the maximum rotation rate of the wheel. LED's 11 and 12 are directed to emit such pulses of electromagnetic radiation by supplying corresponding pulsed currents thereto over interconnections 23 and 24 from operation and processing system 21. Photodetector 18 provides electrical output signals at its output having varying magnitudes corresponding to the emitted electromagnetic radiation pulses insofar as they are reflected from the circular concentric band in surface 15 to that photodetector. That is, there are pulses in the photodetector output waveform for each electromagnetic radiation pulse reaching that photodetector. These pulses in the output signal of the photodetector are what the signal processing circuitry in system 21 operates on to determine the number of rotations, and the direction thereof, of meter eddy wheel 16. A similar system is shown in U.S. Pat. No. 5,442,281 to M. Frisch and A. Naumaan entitled "Method and Apparatus for Deriving Power Consumption Information from the Angular Motion of a Rotating Disk in a Watt-Hour Meter" which is hereby incorporated by reference herein.
However, this rotation monitoring process is subject to condition changes and disturbances which can affect the accuracy of the result obtained for the number of rotations and the directions thereof. The pulse portions of the photodetector electrical output signal corresponding to the reflected radiation pulses impinging on photodetector 18 are detected as occurring with respect to other portions of the signal through the magnitudes thereof exceeding a threshold value. That threshold value is selected to be between the expected output pulse magnitude corresponding to radiation pulses reflected from the least reflective portion of the eddy wheel concentric band outside low reflectivity mark 17 and the output signal value expected for radiation pulses reflected from the most reflective version of low reflectivity mark 17.
Yet, electronic component parameter variation from batch to batch, the circuit operating conditions such as temperature, mounting alignment between LED's 11 and 12 and photodetector 18, and electronic component aging can lead to considerably different magnitudes in the pulses occurring in the output signal of photodetector 18. Thus, magnitudes in the output signal from photodetector 18 may shift in a direction with the result that the threshold value no longer sufficiently discriminates between output signal magnitudes for radiation pulses reflected from the non-reflective mark and those reflected from the rest of the concentric band.
Further complicating the possibility of shifts in the output signal of photodetector 18 is the occurrence of electrical "noise" which may occur in the output signals of photodetector 18, perhaps primarily due to the varying reflectivity of the concentric band of wheel 16 at locations other than the edges of low reflectivity mark 17, and may also occur in the circuitry of operation and processing system 21. All of such noise can lead to further shifts in the signal with respect to the threshold value, shifts which are to a substantial degree random in nature. In addition, external attempts may be made to tamper with the meter, such as through use of external electromagnetic radiation sources, to cause operating and processing system 21 to provide erroneous results with respect to the amount of electrical power consumed. Thus, there is desired an implementation for operation and processing system 21 which can operate LED's 11 and 12 and photodetector 18 so as to provide proper rotation counting even in the face of such condition change and disturbance possibilities.